Well I gotta say, I greatly missed out on these in my first playthrough..

Yes, they are hard to do. Yes you have to have a full compliment of capable mechs of all shapes and sizes to meet the tonnages, but BOY is it worth it..

You get 50% more salvage shares, and that means you can easily get good and expensive mechs that you, would normally not be able to get, even if you're a hero with the associated faction.

I've also learned a simply trick to doing some of these missions if you're not in great shape with available mechs..

Go for MMOps with one demolition mission, and then "discard that one" - by "discard" I mean get a fast light or medium mech, like a Phoenix Hawk (especially the machinegun capable version), and save your other, tougher mechs for other mission types where good and expensive enemy mechs are likely to appear.

Then simply go full throttle, and completely ignore enemy mechs, taking out the base as quickly as possible using the machineguns and stomping through.

I guarantee you you will still have most of your armor at the end od the mission (if you don't mess up in a grand way).

And you will be one mission closer to using your heavy hitters to collect some big, expensive salvage..